1. Field of the Invention
The present invention relates to control actuators, and in particular, to trim actuators for use in aircraft flight control systems, such as helicopter flight control systems.
2. Background of the Prior Art
Automated steering mechanisms and flight control systems for aircraft have been around for many years. An early automatic pilot for helicopters is disclosed in U.S. Pat. No. 2,479,549 issued Aug. 23, 1949 to Ayres et al. The Ayres electrical control system can be overridden so that the aircraft can be operated manually. In Ayres, an aneroid bellows controls an electric motor for adjusting and maintaining altitude, a directional gyro controls an electric motor for adjusting and maintaining heading, and an attitude gyro controls two electric motors for adjusting and maintaining attitude.
A steering mechanism utilizing a servo motor is disclosed U.S. Pat. No. 4,004,537 issued Jan. 25, 1977 to Nilsson. The Nilsson steering mechanism is designed primarily for boats. In Nilsson, rotation of a wheel is passed through a cable to a ball screw cylinder, where the rotation is transformed into linear movement of bar. The rotatable element in the ball screw cylinder is connected to a servo motor, via gears and a transmission. When the cable rotates faster than the servo motor, the transmission decouples the motor and allows the wheel and cable to override the servo motor. The transmission in Nilsson may be a double-acting free-wheeling clutch.
A motor for use in an aircraft flight control system is disclosed in U.S. Pat. No. 5,233,252 issued Aug. 3, 1993 to Denk. Denk contemplates using a two-pole permanent-magnet rotor and a toothless stator motor. A pair of these motors are incorporated into a flight control system replacing hydraulic or electric motors and potentially eliminating the need for an additional friction brake. The motors have a detent, or holding, mode, wherein they act as brakes, resisting back-driving forces acting on the flaps. In Denk, flux gates are inserted by a solenoid and its plunger into a space between the stator core elements. When the motor is not in use, the plunger is retracted by springs, and the flux gates are removed from the spaces. The permanent-magnet rotor simultaneously rotates to align its magnetic axis with the spaces. Once in this preferred orientation, the permanent-magnet rotor resists imposed torque loads. This is the preferred mode, i.e. holding, for the application to which the Denk patent is directed. The need for a solenoid, spring or other mechanism to activate/de-activate the function is an undesirable complication.
With respect to helicopter flight control systems, the prior art trim actuator devices typically include an electrically-actuable motor, a gear assembly, a brake device for locking the position of the rotor of the electrically-actuable motor, a clutch for mechanically controlling the interaction of the electrically-actuable motor and the gear assembly. Additionally, in the prior art devices, an eddy-current damping system is sometimes provided with a permanent magnet member carried by the rotor of the electrically-actuable motor. When a stepper motor is utilized for the electrically-actuable motor, the brake component can be eliminated; however, prior art devices all require some type of clutch to interface mechanically the electrically actuable motor and the gear assembly.